This invention relates generally to waveguide apparatus and methods, and, more particularly, relates to marginally dispersive ultrasonic energy radiating buffer rods.
Apparatus and methods for non-invasive ultrasonic interrogation and testing of materials and structures have become increasingly significant in a wide range of industrial and scientific settings, including aerospace, automotive, and industrial process control applications. Ultrasonic measurement systems are utilized, for example, in measurement of liquid flow rate in conduits, to detect the presence or level of fluid in containers, for evaluation of material thickness, and for determination of fluid flow velocity, density, or temperature.
Many applications for ultrasonic measurement systems require avoidance of disturbance to the physical process or component to be monitored. Other applications involve aggressive physical environments, such as extreme temperatures or pressures which may pose hazards to transducer materials. In particular, ultrasonic measurements of the properties of solids at elevated temperatures, on the order of 1000 degrees Celsius, is much more difficult than at room temperature, because such temperatures exceed the Curie point of most piezoelectric materials and also exceed the destruction temperature of the couplants commonly used near room temperature.
A variety of techniques and special materials have been proposed or developed to address the problems created by the need for measurements at extreme temperatures. One such solution involves the employment of a buffer rod, or waveguide, which can isolate the transducer from direct exposure to extreme temperatures and thermal shock, and reduce heat transfer from the material to be measured. For example, Frederick, 1948, used a long buffer rod, notched at one end, to separate the specimen from the transducer, permitting speed-of-sound measurement of longitudinal waves in the MHz range in solids at elevated temperature. Subsequently, a number of other investigators employed buffer rods in various forms, including buffer rods which were threaded to suppress spurious echoes due to sidewall reflection and mode conversion. Bell, 1957, employed an elongated buffer rod to convey extensional waves, and later, torsional waves, to and from a small diameter specimen. In Bell's experiments, waveguide diameters were typically between one and a few millimeters, and the selected acoustic frequency was approximately 100 kHz.
Industrial applications, such as steel processing, often require measurement of the propagation of ultrasonic compressional waves, or other ultrasonic waves, through media being processed or stored at extreme temperatures. At high temperatures, signal attenuation due to classical and internal effects is usually much higher than at room temperature. If the media under test are fluids, there may also be considerable turbulence, further increasing the observed attenuation. Past research has demonstrated that interrogation signal frequencies on the order of 100 kHz provide a good compromise between limiting attenuation to a low level, such that sound waves can be propagated efficiently through the test medium, while avoiding the low frequency acoustic background noise often present in industrial operations. The 100 kHz frequency range is accordingly utilized for illustrative purposes in the examples of the invention discussed hereinafter.
For a waveguide that is slender with respect to wavelength, the compressional or extensional wave velocity collectively referred to hereinafter as longitudinal c.sub.L is given by EQU c.sub.L =(E/.rho.).sup.0.5
where E=Young's modulus and .rho.=waveguide material density. FIG. 1, which depicts wave velocity as a function of waveguide diameter, indicates that, for diameters small compared to wavelength, as the diameter of the waveguide increases, the wave velocity decreases to EQU c.sub.L (E/.rho.).sup.0.5 [1-(.pi..sigma.a/.lambda.).sup.2 ]
where d is waveguide diameter, a=d/2, .lambda. is wavelength and .sigma. is Poisson's ratio. Wave velocity eventually reaches a minimum velocity approximately equal to the Rayleigh velocity c.sub.R at EQU a/.lambda.=1 (approx.)
according to Tu et al., J. Acoust. Soc. America. 27, pp. 550-555, (1955).
Waveguide theory predicts that acoustic energy propagating through a waveguide can be dispersive in certain circumstances. In accordance with waveguide theory, when the phase velocity of the components of an acoustic signal is a function of frequency, propagation is said to be dispersive. A pulse or packet of sound waves propagating dispersively will separate into a number of different modes of propagation, each having a different group velocity dependent upon the frequency of the acoustic energy. The number of modes, in turn, depends upon the frequency of sound and the radius of the waveguide. In contrast, in a nondispersive waveguide, sound speed is independent of frequency. The dispersion relation is found in several texts, e.g., Kolsky, Stress Waves in Solids. p. 59, Eq. 3.60 (1963). A recent reference on dispersion in both isotropic and anisotropic waveguides is N. C. Nicholson, W. N. McDicken and T. Anderson, in Ultrasonics Vol. 27, pp 101-106 (March, 1989).
Tu et al., J. Acoust. Soc. of America, 27, pp. 550-555, (1955), indicate that waveguide diameter d must exceed about 5.lambda. in order for longitudinal waves to propagate in solids nondispersively. Conventional practice suggests that waveguides for materials testing and process monitoring should be nondispersive to enable precise measurement of signal arrival time, and that dispersive waveguides generate an unacceptable degree of signal "smear" or pulse distortion.
Experiments conducted by Bell, 1957, and Gelles, 1966, to develop nondispersive waveguides, demonstrated that a single buffer rod, or a bundle of thin fibers, respectively, could provide a nondispersive buffer. The fiberacoustic nondispersive bundle is discussed in Gelles, J. Acoust. Soc. of America 39 (6), pp. 1111-1119 (1966). Gelles describes a bundle in which the fibers are so slender as to be nondispersive.
However, past attempts to assemble a large number of very slender fibers into a practical probe, for testing red hot steel or other materials having scaled or irregular surfaces, have presented serious technical problems. These difficulties are due to the large number of fibers required for adequate signal transmission, and the low flexural strength of conventional waveguide structures. Ultrasonic measurement systems typical of the prior art do not include elements for pressing fibrous waveguides in a bundle against a test medium, and fail to disclose means for withstanding, without leaking, the pressure of an adjacent fluid at high pressure.
Additionally, certain conventional ultrasonic interrogation systems do not provide elements for resisting waveguide buckling. For example, Rogers and Miller, in IEEE Trans. Nucl. Sci. NS-29 (1), pp. 665-668 (February 1982) disclose a stainless steel diaphragm of thickness 50 micrometers--approximately 0.1% of a wavelength--as a low-reflectivity feedthrough penetrated by a welded-in slender waveguide. The Rogers and Miller effort was directed toward determination of liquid level, density and temperature. Their apparatus, utilizing pulse-echo techniques, required highly precise measurement of the time intervals between echoes. These echoes were generated through intentionally introduced discontinuities in the waveguide, rather than in an adjacent medium. By supporting the diaphragm near the waveguide, the unsupported area was kept small, thereby minimizing the force due to hydrostatic pressure. The waveguide itself was suspended vertically and kept straight and in tension by means of a weight element.
The Rogers and Miller apparatus presented no compressive load tending to buckle the waveguide. Waveguide diameter and support elements were therefore not selected or configured to withstand buckling. In accordance with conventional practice, moreover, the cross sectional dimensions of the waveguide were small, compared to wavelength, in order to minimize dispersion. These small cross sections typical of the prior art limit the compressive force which the waveguide can withstand.
The maximum force F.sub.max that a 50 micrometer diaphragm of 10 millimeters diameter can support is readily calculated. Assuming a yield strength in be EQU (50 .mu.m)(.pi.)(10 mm)=500(.pi.)*10.sup.-9 m.sup.2 =0.002 in..sup.2
then EQU F.sub.max =30,000*0.002=60 pounds
Thus, while a diaphragm 10 millimeters in diameter can have a 30,000 psi yield strength and can sustain a hydrostatic pressure of nearly 500 psi over its area of approximately 0.125 square inches, it would fail if utilized for applying significant coupling force to a waveguide.
As a numerical example of coupling forces required at room temperature, Crecraft, J. Sound and Vibr. 1 (4), pp. 381-387 (October 1964), indicates that approximately 20,000 psi is required to maximize coupling between machined steel surfaces. For a waveguide of diameter d=0.125 inches or about 3 mm, this implies a required coupling force of EQU F=(20,000 psi)(.pi./4)(0.125.sup.2 in..sup.2)=245 pounds
which greatly exceeds the 60 pound limit calculated above for F.sub.max. Furthermore, a steel waveguide having an unsupported length of at least one foot, and a diameter of 0.125-inch, would buckle at loads far less than 245 pounds, or even 60 pounds.
Conventional waveguide seals and support elements generally do not provide enough structural rigidity for radiating ultrasonic energy from an extensional wave source into fluid media which are at high pressure or into solid media at red hot temperature, which have scaled surfaces, and which accordingly require coupling pressures on the order of thousands of psi or multiple Mpa. (One MPa is approximately equal to 150 psi.)
Moreover, when measuring propagation in media at elevated temperature, the use of longitudinal waves in the MHz range introduces high attenuation, and often requires threading of the buffer rod along most of its length to suppress sidewall echoes. Conversely, conventional application of nondispersive extensional waves in the 100 kHz range implies the utilization of a source diameter at the contact point that is so small as to be inefficient as a radiator in many cases
It is thus one object of the invention to provide improved waveguide structures having an efficient aperture for radiating from an ultrasonic wave source into media which are at high pressure or temperature or which require high coupling pressure
It is another object of the invention to provide waveguides having enhanced impedance matching and optimum ultrasonic energy transmission into a wide range of test media.
It is a further object of the invention to provide such waveguides which can withstand high coupling pressures without buckling.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.